Sample Preparation for SERS

Solid-state 13C NMR has been shown to be a more effective analytical tool for demonstrating the formation of P-sheets in polypeptides and proteins, because the isotropic 13C NMR chemical shifts of carbon atoms in proteins are sensitive to the P-sheet s secondary structure. It is well established that SF conformations are dependent upon the species of silkworms and conditions of the sample preparation. In particular, has been reported that fibroin from Bomhyx mori adopts two dimorphic structures, silk I and silk II. The silk II form is identified by the C chemical shifts of glycine (Gly), serine (Ser), and alanine (Ala) that are indicative of P-sheets, while the silk I form produces chemical shifts that are associated with a loose helix or distorted P-tum. However, when compared with silk II, the less stable silk I shows a relatively unresolved structure, and the conformation of the soluble form of SF rapidly undergoes a transition to the insoluble silk II conformation. 

To prepare naturally occurring nanostructures for SERS, the samples must be sectioned, cleaned, mounted onto a suitable medium (such as a glass substrate), and... 

The use of solid silver nanoparticles indicates the 10-fold enhancement of Raman signals due to presence of art pigments. The enhancement of the order of magnitude is not SERS effect. However it is high enough to identify very small quantities of such specific samples as art pigment from canvas paintings. The proposed technique does not require any complex procedures for sample preparation. 

The silver deposition on the surface of PS was carried out by the immersion plating in AgNOs solutions. Samples for SERS measurements were prepared by incubation of Ag-PS substrates in the analyte solution for two hours. SERS spectra were obtained with excitation at 457.9 nm. 

Parliment T.H. (1986) Sample preparation techniques for gas-liquid chromatographic analysis of biologically derived aromas. Am. Chem Soc. Symp. Ser. 317, 34-52. 

Iyengar GV and Sansoni B (1980) Sample preparation of biological materials for trace element analysis. In Elemental Analysis of Biological Materials, IAEA Tech Rep Ser 197, pp. 73-101, International Atomic Energy Agency, Vienna. 

The examples of SERS and SERRS measurements in the biochemical field reviewed in this article were selected to illustrate the sensitivity, molecular specificity of adsorption process, accuracy, ease of sample preparation, and significant manifold apphcations of Raman analysis by SERS and SERRS for biomolecules in the adsorbed state. 

Because of the huge signal enhancement, SERS is particularly useful for trace analysis and for in-situ investigations of various interfacial processes or of mono-layers adsorbed on metals. However, sample preparation is a rather tedious procedure. For this reason, SERS is still more an academic tool rather than a routine analytical instrument. Some applications of SERS are given in [31]. 

Aliquots of analyte solutions are added to colloidal solutions or deposited onto the metal island surfaces thin layer spectra are obtained after diluting the sample on the surface using an appropriate solvent. All the spectra are scanned after complete solvent evaporation. Samples for SERS and SEIRA are also prepared by vacuum evaporation of controlled mass thickness. 

Preparing metal surfaces requires expertise and can be expensive. Commercial sources for SERS substrates, colloids, and sample holders are available from companies such as Thermo Fisher Scientific, Renishaw, and Real-Time Analyzers (www.rta.biz), which focuses on high-throughput needs with SERS 96 well plates. 

Bulk Copper Preparation of bulk copper surfaces (samples may consist of rods, foils, or disks) includes polishing, rinsing, and in some cases degreasing. When samples are used for SERS, a roughening step is added. This was achieved by... 

Fig. 6.2 Basic scheme of intracellular SERS experiment, a Scheme for the delivery of Au NPs into eukaryotic cells from the culture medium. Parallel samples were prepared for biological assays, electron microscopy and Raman microspectroscopy, b Cell lines NIH/3T3 upper panel) and J774 lower panel) after several hours of incubation with Au NPs (scale bar 20 pm) (adapted with permission from Kneipp 2006. Copyright 2006 Springtar)...

The main features of PC are low cost, need for small sample amount, high level of resolution, ease of detection and quantitation, simplicity of apparatus and use, difficult reproducibility (because of variation in fibres) and susceptibility to chemical attack. Identification of the separated components is facilitated by the reproducible Rj values. Detection methods in PC have been reviewed [368]. Fluorescence has been used for many years as a means of locating the components of a mixture separated by PC or TLC. However, also ATR-IR and SERS are useful. Preparative PC is unsuitable for trace analysis because filter paper inevitably contains contaminants (e.g. phthalate esters, plasticisers) [369]. For that purpose an acceptable substitute is glass-fibre paper [28]. 

The observation and understanding of SERS are clearly very important developments in the study of surface chemistry and surface physics. The combination of molecular information and extraordinary sensitivity provides a valuable probe of surface structure and behavior. Out of the broad study of SERS by both chemists and physicists have emerged several approaches to using SERS for chemical analysis. A common analytical situation involves preparation of a SERS active substrate by one of several methods, then exposure of the substrate to a liquid or gaseous sample. Subsequent Raman spectroscopy of the adsorbed layer provides the analytical signal, enhanced by whatever chemical or field enhancement is provided by the adsorbate-substrate interaction. The current and next section are not intended to address SERS substrates comprehensively, but several of analytical interest are described. 

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